Bug 975996: Add _PyTime_DoubleToTimet to C API

Bug 975996:  Add _PyTime_DoubleToTimet to C API
New include file timefuncs.h exports private API function
_PyTime_DoubleToTimet() from timemodule.c.  timemodule should export
some other functions too (look for painful bits in datetimemodule.c).

Added insane-argument checking to datetime's assorted fromtimestamp()
and utcfromtimestamp() methods.  Added insane-argument tests of these
to test_datetime, and insane-argument tests for ctime(), localtime()
and gmtime() to test_time.